The multiplicity of feedback mechanisms that influence cardiac performance places the heart at the center of a complex control network. The neurohumoral axis includes the autonomic nervous system, consisting of sympathetic and parasympathetic branches, and numerous circulating hormones such as catacholamines, angiotensin, and aldosterone. Neural reflex arcs originating from pressure and stretch receptors, which directly measure mechanical hemodynamic status, modulate the neurohumoral axis. Similarly, chemoreceptors respond to changes in CO2, pH, and O2, which assesses cardiopulmonary function. The neurohumoral system influences cardiac performance at the level of the cardiac electrical system by regulating heart rate and the conduction velocity of electrical depolarizations. It also influences cardiac performance at the mechanical level, by controlling contractility, that is, the effective vigor with which the heart muscle contracts. Conventional cardiac monitors, such as defibrillators, pacemakers, Holter monitors, and cardiac event records, are tailored for the diagnosis and/or therapy of abnormalities of the cardiac electrical system. In contrast, heart failure is a disease of the cardiac mechanical system: it is primarily a failure of the myocardium to meet the mechanical pumping demands required of it. In monitoring the status of a heart failure patient, measuring the mechanical hemodynamic variables is clearly desirable. Examples of mechanical hemodynamic variables include atrial, ventricular, and arterial pressures, and cardiac output (volume of blood pumped into the aorta per unit time). However, because of the complex feedback network that monitors and controls cardiac performance, measuring variables that do not directly reflect the mechanical performance of the heart is also useful. In this way, measuring cardiac electrical activity to assess heart rate variability (described below) allows one to infer the state of the autonomic nervous system, which allows one to infer information about the hemodynamic status of a heart failure patient. Similarly, recognition of Cheyne-Stokes respiration (described below) via respiratory pattern analysis, hemoglobin saturation analysis, and blood gas analysis allows one to detect the presence of pulmonary edema, and thereby detect an acute heart failure exacerbation, though none of these parameters directly measure mechanical hemodynamic status. It would be useful to provide additional and/or alternative ways for monitoring the state of a patient's autonomic nervous system, which are not dependent on heart rate.
One approach to frequent monitoring of heart failure patients that has been proposed is the daily acquisition of the patient's weight and responses to questions about subjective condition (Alere DayLink Monitor, Alere Medical, Inc., San Francisco, Calif.). The simplicity and noninvasive embodiment of this approach are desirable features. However, both the amount and the sophistication of the objective physiological data that can be acquired in this way are quite limited, which consequently limits the accuracy of the system. Furthermore, the system requires the active participation of the patient, who must not deviate from the precise data acquisition routine or risk introducing confounding factors into the acquired data.
Some of these limitations have been addressed by the development of an implantable system that monitors hemodynamic status (Medtronic Chronicle, Medtronic, Inc., Minneapolis, Minn.). While this system potentially avoids the need for active patient participation, it relies on an intravascular sensor placed in the right ventricle of the heart. This approach is consistent with the prior art for implantable hemodynamic status monitoring, which has to date focused on intravascular or intramyocardial instrumentation. Examples include U.S. Pat. No. 5,454,838 in which Vallana et al. teach placement of a sensor on the myocardial wall using an intravascular approach. In U.S. Pat. No. 5,496,351, Plicchi et al. propose placing a sensor within the myocardial wall. Mortazavi in U.S. Pat. No. 5,040,538 and Cohen et al. in U.S. Pat. No. 4,815,469 describe placement of an optical sensor within the right ventricle. In the context of hemodynamic assessment for arrhythmia discrimination, Cohen and Liem (Circ., 1990, 82:394–406) study the effectiveness of a pressure transducer placed in the right ventricle. Clearly, powerful information about hemodynamic status can be obtained using intravascular instrumentation. However, intravascular or intramyocardial instrumentation carries significant risks to the patient, including increased perioperative morbidity and mortality, and increased long-term risks such as stroke and pulmonary embolism. Furthermore, intravascular instrumentation can only be performed by extensively trained specialists, thereby limiting the availability of qualified physicians capable of implanting the device, and increasing the cost of the procedure. Finally, because of the added patient risks and greater physical demands of an intravascular environment, the intravascular placement of the sensor increases the cost of development, manufacturing, clinical trials, and regulatory approval.
Though not directly related to hemodynamic status monitoring, extravascular sensing of cardiac electrical activity is known in the art. Early generations of implantable pacemakers and defibrillators relied on epicardial placement of sensing electrodes. Epicardial electrodes still see use in special patient populations. Extrathoracic sensing of cardiac electrical activity is also possible, which avoids the need for direct contact with the heart, and thus decreases the difficulty of the implant procedure and reduces the risk of perioperative complications. An example of this approach is the Reveal Insertable Loop Recorder (Medtronic, Inc., Minneapolis, Minn.), a cardiac event recorder configured for short-term implantation. As a temporarily implantable recorder, it overcomes some of the technical difficulties associated with conventional externally worn recorders of cardiac electrical activity. Two general types of externally worn recorders are Holter monitor recorders, which record continuously for an extended period of time, and cardiac event recorders, such as the King of Hearts (Alaris Medical Systems, San Diego, Calif.), which use a loop memory to retain the most recent history of cardiac electrical activity. Both these approaches require surface contact electrodes which are cumbersome and inconvenient for the patient, and more susceptible to motion artifact than an implanted electrode. However, like conventional cardiac event recorders and continuous Holter monitor recorders, the Reveal Insertable Loop Recorder is designed for short-term use as a diagnostic aid. More importantly, it requires active patient participation; when the patient recovers from syncope, or becomes aware of symptoms, he must signal to the event recorder by means of an Activator that the recent data should be retained in long-term memory for later review by a physician. After diagnosis the Reveal Insertable Loop Recorder is explanted from the patient. Thus the Reveal is intended for short-term recording for diagnostic use, is limited to recording the electrical activity of the heart, and does not attempt to measure or quantify the hemodynamic status of the patient beyond screening for cardiac arrhythmias.
An extension of the short-term recorders just described is the Implantable Ambulatory Electrocardiogram Monitor described by Nappholz et al. in U.S. Pat. No. 5,113,869, incorporated herein by reference. This device is designed for chronic extravascular implantation. In contrast to cardiac recorders, it performs analysis on the electrocardiogram signal in order to predict imminent cardiac arrhythmias and to detect cardiac ischemia. Like the cardiac recorders, it is capable of storing raw ECG data for later review by a physician. This feature, along with the record of arrhythmic events it detected, allows the physician to tailor pharmacologic therapy. In addition, Nappholz et al. mention the use of transthoracic impedance for minute ventilation, ultrasound transducers for arterial pressure, or other sensors to allow discrimination of arrhythmias from normal cardiac rhythms caused by exertion or physiologic stress.
While the Holter monitor recorder, the Reveal Insertable Loop Recorder, and the Implantable Ambulatory Electrocardiogram Monitor provide important clinical utility in recording and monitoring cardiac electrical activity, none is designed to monitor hemodynamic status. Indeed, cardiac electrical activity does not, by itself, provide unambiguous information about hemodynamic status. By sensing only cardiac electrical activity, these devices are unable to distinguish between, for example, a hemodynamically stable cardiac rhythm and Pulseless Electrical Activity (PEA), a condition in which the heart is depolarizing normally, and thus generating a normal electrical pattern, but is not pumping blood. Furthermore, these devices are unable to recognize or quantify subtle changes in the patient's hemodynamic status. What is needed is an extravascular, hemodynamic monitor designed for chronic use.
While much of the prior art has focused on intravascular instrumentation, as discussed above, some proposal has been made to incorporate physiologic sensors into the implantable cardiac device itself. Fearnot in U.S. Pat. No. 5,040,533 teaches placement of a generalized window in the housing of the cardiac device. The window might be transparent to facilitate the transmission of light or flexible to facilitate pressure transduction. While the convenience, from the clinician's perspective, of incorporating the sensors into the housing of the cardiac device is an obvious advantage, the technical difficulty in maintaining a hermetic seal between two different materials, particularly in a chronically implanted device, is equally obvious to one skilled in the art. The technical challenge is made more difficult by the greatly increased circumference, relative to that of standard feed-through connections known in the art, of the boundary between the window and the device housing. What is needed, therefore, is a method of placing a hemodynamic sensor in or on the device without compromising the integrity of the hermetic enclosure.
Prutchi et al., in U.S. Pat. No. 5,556,421 propose placement of a sensor within the header of a cardiac device. While this is an obvious solution for devices that have external leads requiring headers, it presupposes the existence of a header, and therefore does not address the implantable device that lacks a header. Furthermore, while appending a header to one end or pole of an implantable device is an efficient solution when external leads are required, appending a header-like sensor unit to one end or pole of a device not otherwise requiring a header, where the sensor unit is itself, like a header, the full-thickness of the device, is an inefficient use of volume. Thus, the approach of Prutchi et al. used in a device that doesn't otherwise require a header would be to append a header or a header-like sensor unit to one end or pole of the device, but this would unnecessarily increase both the volume and the expense of the device. A further disadvantage of placing a sensor in a header is that it does not necessarily provide for the optimal signal transduction of a particular sensor. For example, the performance of the optical sensor described in the above referenced U.S. Pat. No. 5,556,421 would be so severely degraded by direct transmission of light from source to detector that one skilled in the art would question the functionality of the proposed solution. In addition, placement in a rigid epoxy header is simply not an option for some sensors, such as sound sensors, because of the dramatic degradation in the signal-to-noise ratio the rigid header would impose. What is needed is a method of incorporating a hemodynamic sensor into a implantable device, providing it optimal access to the external milieu so that the signal of interest is optimally transduced, maintaining the hermetic enclosure provided by the device housing, and minimizing the added volume that the sensor imposes. In addition to improving hemodynamic sensors, it would also be useful to provide new uses for such sensors.